A polypropylene which is a general purpose resin and a copolymer of propylene with another copolymerizable monomer (hereinafter they will collectively be referred to as a “polypropylene resin”) are available relatively at a low cost and have favorable properties such as moldability, heat resistance, solvent resistance, mechanical properties and appearance and are thereby processed into various molded products and used in various fields. However, a polypropylene resin is basically composed of a saturated hydrocarbon and is thereby poor in chemical reactivity and has low polarity, and accordingly improvement in adhesive properties, coating properties and printing properties has been desired.
As a method for improving them, a method of grafting, to a polypropylene resin, an unsaturated carboxylic acid or its anhydride, represented by e.g. maleic anhydride, to modify the polypropylene resin, has been known. Such an acid-modified polypropylene resin is produced by graft reaction of an organic acid component, using, as an initiator, radicals generated by an organic peroxide, heat decomposition or the like.
However, it has been known that the graft reaction is a competitive reaction between the graft reaction of an acid component and β-cleavage reaction of the polypropylene molecular chain, and since the β-cleavage reaction rate is generally higher than the graft reaction rate, improvement in the grafted amount involves a decrease in the molecular weight resultingly. Such a polypropylene resin having its molecular weight decreased during the graft reaction has impaired physical properties such as adhesive strength and impact resistance, and accordingly from the viewpoint of improvement in physical properties, development of an acid-modified polypropylene resin having improved balance between the grafted amount and the molecular weight has been desired, and various proposals have been made (Patent Documents 1 and 2).
In recent years, as requirements for resin materials in various applications are increasingly high, development of a composite material having a plurality of properties by bonding such different materials differing in characteristics is in progress. To develop such a composite material, a high quality adhesive to bond different materials is required, but usually a polyolefin type resin such as a polypropylene has low polarity and thereby has very poor adhesive properties with a polar resin, and thus development of an adhesive capable of bonding a polyolefin type resin and a polar resin with sufficient adhesive force has been desired.    Patent Document 1: JP-A-2002-020436    Patent Document 2: JP-A-2003-183336